Processes to separate hydrocarbon mixtures into component portions are an important necessity in industry and can employ a wide variety of techniques. Some separation processes involve contacting the hydrocarbon mixture with a membrane through which one component type is able to permeate. For example, U.S. Pat. No. 5,039,422 disclosed using a urea polymer chain extender with a compatible second prepolymer membrane to separate aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons. Other separation processes involve solvent extraction such as using tetrahydrothiophene 1,1-dioxide to recover high purity aromatic hydrocarbons such as benzene, toluene, and xylenes from hydrocarbon mixtures, see Wheeler, T. In Handbook of Petroleum Refining Processes; Meyers, R. A., Ed.; McGraw-Hill Book Company: New York, 1986, Chapter 8.4. A third type of separation process includes using a solid adsorbent to selectively adsorb desired components from the mixture. The components are later desorbed and recovered; see U.S. Pat. No. 4,048,111.
The separation of mixtures of hydrocarbons according to whether components are aliphatic or aromatic, and separations of the aliphatic hydrocarbons according to whether components are saturated or unsaturated are examples of separation processes that may be carried out using a solid adsorbent. The term aliphatic means those compounds which are not aromatic. Typically the solid adsorbents used in industry are zeolites or molecular sieve materials such as those described in U.S. Pat. Nos. 4,036,744 and 4,048,111. The present invention expands the range of useful solid adsorbents to effect hydrocarbon separations to include alkylene-bridged polysilsesquioxanes. Specific organically-bridged polysilsesquioxane materials and their preparation have been disclosed in Shea, K. J., Loy, D. A. Chem. Mater. 1989, 1, pp. 572-574; Shea, K. J., Loy, D. A., Webster, O. J. Am. Chem. Soc. 1992, 114, pp. 6700-6710; Shea, K. J., Loy, D. A., Webster, O. Mater. Res. Soc. Symp. Proc. 1990, 180, pp. 975-980; Shea, K. J., Loy, D. A., Webster, O. Polym. Mater. Sci. Eng. 1990, 63, pp. 281-285. Specific alkylene-bridged polysilsesquioxanes and their preparation have been disclosed in Oviatt, H. W., Shea, K. J., Small, J. H. Chem. Mater. 1993, 5, pp. 943-950 and Small, J. H., Shea, K. J., Loy, D. A. J. Non-Crystalline Solids 1993, 160, pp. 234-246. This art teaches that organic groups can be introduced at regular intervals in an inorganic silicate framework, thus forming a three-dimensional organic-inorganic hybrid silicate-like polymeric material, also called an organically-bridged polysilsesquioxane. A two-dimensional representation of the well-known inorganic silicate framework is shown in I, and an analogous representation of the organically-bridged polysilsesquioxane where the represents the organic bridging group is shown in II. Of course, frameworks I and II, in reality, extend to form a three-dimensional, continuous, amorphous solid. ##STR1##
The specific alkylene-bridging groups disclosed in the art include ethylene, tetramethylene, hexamethylene, octamethylene, decamethylene, and tetradecamethylene. In view of the prior art, alkylene-bridging groups of trimethylene, pentamethylene, heptamethylene, nonamethylene, undecamethylene, dodecamethylene, tridecamethylene and their preparation are readily apparent to those skilled in the art. Organically-bridged polysilsesquioxanes containing these bridging groups are termed alkylene-bridged polysilsesquioxanes. One stated objective of the disclosed work was to provide molecular level control of the morphology of the framework, another was to provide a new chromatographic support, and a third was use in optical applications. However, applicant has found that these materials perform as adsorbents for saturated, unsaturated aliphatic, and aromatic hydrocarbons at low temperatures. Furthermore, applicant has discovered that adsorbed saturated, unsaturated aliphatic, and aromatic hydrocarbons may be desorbed from the alkylene-bridged polysilsesquioxanes using environmentally preferred saturated hydrocarbon desorbents.